Apparatus for measuring and of controlling the gap between polymer sheet cooling rolls

ABSTRACT

Heating and extruding a polymeric sheet having a predetermined thickness and introducing it into and through a gap between cooling rolls, measuring directly by laser beam and without reflection the gap between the cooling rolls, and controlling the gap to control minimal pressure on the softened and heat-plastified sheet.

FIELD OF THE INVENTION

[0001] This invention relates to apparatus and method for accurately anduniformly cooling a preformed polymeric sheet. It further relates toapparatus and method for extruding and cooling a preformed polymericsheet wherein the gap between adjacent rolls is measured and controlledfor accurately and gently cooling the polymeric sheet with minimumstress.

DESCRIPTION OF THE RELATED ART

[0002] Polymeric extruded or coextruded sheets are usually extruded outof a slit die of appropriate width, as indicated in the Frank R. NisselU.S. Pat. Nos. 3,883,704, 3,918,865, 3,940,221, 3,959,431 and 4,533,510,for example. The hot sheet is then cooled by passing it through a pairor a series of temperature-controlled rolls. Although various rollnumbers and arrangements may be used, three rolls are often used,sometimes in planar alignment with each other. The gaps between therolls are desired to be precisely adjusted according to the desiredfinal sheet thickness. This precision is necessary for a variety ofreasons, including elimination of air entrapment between the rolls,which causes adverse or uneven heat transfer or cooling. The rollstypically apply embossing to provide a high quality surface impressionon the sheet.

[0003] Traditionally, the sheet die exit is horizontal and the sheetruns through a cooling roll stack at any angle, either upwardly ordownwardly. Vertical, horizontal or various angled roll stacks may beused.

[0004] For various purposes, the art has made efforts from time to timeto measure the gap distance between cooling rolls. Gauging the gapdistance has been achieved mechanically through the use of feelergauges. Unfortunately, feeler gauges lack the precision needed formaintaining optimal cooling and surface impression. They provide only ago/no go type reading without giving accurate quantitative measurements.They further tend to scratch the roll surfaces. Feeler gauges also posethe danger of being accidentally drawn into the rolls and permanentlydamaging them.

[0005] To avoid these dangers, methods have been introduced whereindirect measurement of the gap is taken. This includes measuring thesheet material which passes between the rolls, measuring the distancebetween the mounting blocks used to support the rolls, and measuringreduced diameter barrel portions. These methods, however, do not andcannot account for actual differences in the diameter of the roll,caused by changes in temperature and other factors. An accurate way tomeasure the gap between the rolls would be to measure the gap directly,but the reported attempts have not succeeded.

[0006] Several devices which monitor a sheet are used to influence thethickness of the sheet product. For example, U.S. Pat. No. 3,809,907which issued to Schuller et al on May 7, 1974, discloses a device forcontrolling a bank of viscous steel that is formed on, and builds up infront of, the nip of a pair of spaced rolls. Specifically, the rolls arefirst maintained in close proximity such that the molten steel appliedto an entry side of the rolls forms a bank of hot material against andbefore passing through the rolls. The amount of molten steelaccumulation is measured by a pair of laser emitters and detectors,located at or near the ends of the rolls. But the laser measures theamount of molten steel that is banked up before it flows between therolls, not the actual roll gap.

[0007] Turning now to the field of plastic sheets, extrusion dies withvariable lip portions have been used to form polymeric sheets of varyingwidths and thickness. Once extruded through an adjustable lip die, thepolymeric sheet is substantially at its desired thickness.

[0008] However, there is a demand for cooling of a preformed polymericsheet without excessively stressing the polymeric sheet product.

[0009] There is a need to provide an apparatus for extruding polymericsheet materials with an extrusion die capable of pre-forming polymericsheets of accurately predetermined thickness, and a cooling devicehaving cooling rolls defining a gap of precisely known dimension throughwhich the polymeric sheet passes for the purpose of cooling orembossing, wherein the rolls rapidly and uniformly cool the polymerwhile exerting only minimal pressure on the extruded sheet.

SUMMARY OF THE INVENTION

[0010] This invention includes an extruder and cooling rolls that havean adjustable gap through which a preformed polymeric sheet passes. Thegap between the rolls is accurately measured and controlled to allow therolls to exert only a minimal pressure on the polymeric sheet and toimpart the best surface finish to the sheet while subjecting it to thelowest possible stress level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a conventional plastic sheet extrusion line used forhigh speed production of sheet in rolls, and

[0012]FIG. 2 is an end view of a three-roll stack of polymer sheetcooling rolls arranged for direct roll gap measurement in accordancewith this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] Although the present invention will be hereinafter described inregard to specific embodiments thereof, these embodiments are providedmerely for illustration. This invention is not limited to the specificdetails; its scope is defined in the appended claims.

[0014] Turning now to FIG. 1 of the drawings, showing a conventionalsheet extrusion line, the number 1 is a conventional extruder thatplasticizes and pumps the plastic material to die 1A that extrudes asheet of finite width. 2 is a three roll sheet take-off also knowncolloquially as a calendar. It serves to cool the extruded sheet back tonear room temperature. These three rolls are driven by a variable speedmotor. 3 is an idler roll conveyor positioned to support the sheet whilebeing finally cooled by ambient air. 4 is a set of pull rolls providedto maintain tension on the sheet going over the conveyor 3. These pullrolls are driven by a second variable speed motor.

[0015]5 is a roll winder producing rolls from the pull rolls 4 of finitelength. An indexing two-position turret winder is shown because of itsconvenience, but any other single or multiple type of roll winder can beused.

[0016] Cooling and surface imprinting can be accomplished simultaneouslyusing cooling rolls. The polymeric sheet is hot enough or warm enough tobe susceptible to damage and deformation if not handled gently. For thisreason, the sheet should be introduced to the cooling rolls promptlyafter leaving the die.

[0017] The rolls of the cooling roll stacks are generally made up of aplurality of cooling rolls, turning parallel on axes of rotation. Insome cases three or more cooling rolls are used, as shown for example inFIG. 2. Each roll is designed to be a heat sink for transferring heatfrom the polymeric sheet during the cooling process, without changingthe symmetry and thickness of the sheet. Some or each of the coolingrolls may also be provided with an embossing platen surface, which isalso subject to the dangers earlier discussed.

[0018] Between at least two adjacent rolls, a gap is defined. Two suchgaps are shown at “g” and “g'” in FIG. 2. Each gap performs a vital rolein the production cooling of high quality polymeric sheets.

[0019] In accordance with this invention each gap “g” is preciselymaintained at a desired uniform distance from roll edge to roll edge. Alaser light emitter 15 is provided at the upstream or downstream side ofeach roll gap, and projecting through each gap “g.”0 A laser beamdetector 16 is provided on the opposite side of each gap “g” formeasuring the amount or extent of light passing through the gap. In eachcase the laser light passes through the roll gap itself outside the areacovered by the sheet S. There is a laser on each edge of each roll, atotal of four in FIG. 2. A sensor such as a computer or other device(not shown) is provided for comparing the measured light that passesthrough each gap to a control value corresponding to a gap distance. Thecomparing means may be further connected to a means for controlling thelinear displacement of the rolls with respect to each other to achieveand an accurately maintained gap between the rolls, and to report themeasured value to the operator.

[0020] Each laser is preferably aimed so the laser light hits both rollsurfaces 2, 2′ and 2″, and a portion of the laser light passes betweenthe rolls and through the gap. This ensures accurate direct andnon-reflective light measurement of the thickness of the gap itself.

[0021] This measurement system maybe used continuously throughout theextrusion cooling process. The gap distance maybe adjusted whennecessary, either manually or automatically in view of a lasermeasurement if desired, thereby maintaining the desired gap distancewithout damaging or reducing the thickness of the delicate polymericsheet. Importantly, this allows for real-time adjustment due tofluctuations in the roll diameter caused by temperature fluctuations andother factors.

[0022] The rolls are preferably specifically and precisely set not toexert enough pressure to damage or to change substantially the thicknessof the polymeric sheet. Preferably, the rolls are adjusted from time totime or continuously to maintain a gap which is preset to maintainprecise contact with the polymeric sheet for cooling or also forimprinting the sheet surface with the desired pattern.

[0023] It is important to control the roll gap to a preset or preciselyknown dimension to impart the best surface finish to the sheet whiledeveloping the lowest possible stress level.

[0024] The polymer cooling roll arrangement of FIG. 2 may be used incooling roll stacks having vertical, horizontal, or angular or otherorientations. Preferably, a roll stack will often have three rollsarranged such that their axes of rotation are coplanar. However, manydifferent combinations or arrangements may be used.

[0025] When the polymeric sheet leaves the cooling stack, it has beencooled sufficiently to its final form, and embossed if desired.Subsequently, it is introduced to a takeup spool 5 (FIG. 1), where it isrolled for storage or delivery.

[0026] The method of treatment of the sheet according to this inventionincludes the step of extruding the polymeric sheet to substantially itsfinal thickness and then introducing it into and through the controlleddimension gaps between the cooling rolls. The controlled gap dimensionis of primary importance and control. The roll pressure may vary betweenabout 20 to 500 pounds per linear inch of roll width.

[0027] In a preferred form the gaps between each two adjacent rollersare automatically and continuously monitored by the laser gap monitorand continuously or periodically adjusted to a predetermined roll gap.

[0028] As described, the gap distance is desirably first measured byprojecting a laser at and through the gap and directly withoutreflection to a beam detector or photosensor on the opposite side of thegap. A comparator connected to the detector then compares the receivedsignal to a control signal which may or may not correspond to a desiredgap distance. The comparator then communicates the results to a computerfor linear adjustment of the rolls defining the gap distance. One ormore rolls is then adjusted linearly, accordingly. This process iscontinued preferably throughout the polymer extrusion process, andtherefore maintains exacting standards on the polymeric sheet thickness.

[0029] Knowledge by the operator of strictly controlled gap distances iscritical to the consistent production of quality polymeric sheetmaterial. If the roll gap is too large, or irregular in size or shape,air is occluded between the roll surface and the sheet, resulting inpoor or uneven cooling and poor surface finish. On the other hand, ifthe roll gap is too small, the incoming material cannot pass uniformlycompletely through the gap and this results in accumulation of a bank ofpolymeric material, causing unacceptable surface imperfections andstress in the sheet and other defects. An insufficient roll gap alsocreates added pressure on the polymeric sheet, and may undesirablyeffect the thickness of even a sheet which has earlier been accuratelyformed to its proper and accurate specifications by the extrusion die.

[0030] In accordance with this invention the laser operates without rollreflection. Since the light passes through the tangential point of tworolls, there is no possibility of reflected light. In any case, thedetector is opposite the light source from the rolls so that anyreflection would not be detected.

[0031] The rolls are usually held closed by an adjustable pneumatic orhydraulic pressure system so that they can open up in case of extremematerial pressure to avoid damage to the rolls.

[0032] The ability to maintain a precisely preset roll gap is mostdesirable to produce uniform stress free sheet. Direct roll gapmeasurement according to this invention is the only reliable means ofachieving this.

What is claimed is:
 1. An apparatus for extruding and cooling polymericsheet materials, said apparatus comprising: a polymer sheet extruderhaving a die designed to preform and produce initially soft andheat-plastified polymeric sheets of predetermined thickness; a pluralityof cooling rolls, with a gap between them, and with means forintroducing said sheet into said gap, each said roll having alongitudinal axis of rotation and being capable of cooling aheat-plastified sheet polymeric sheet produced by said extruder; atleast one of said rolls being mounted for selective displacement withrespect to another of said rolls, said rolls being positioned onopposite sides of and adjacent to said gap such that the axes ofrotation of said rolls provide a tangential point for said sheet betweensaid cooling rolls, each opposed roll being mounted in a frame for axialrotation; and a laser source connected for measuring directly andwithout reflection the thickness of said gap, said laser source beingaimed and arranged for transmitting a beam of laser light into anddirectly through said gap and through said tangential point of saidcooling rolls; a laser beam detector arranged in line with said gap on aside directly opposite to said laser source for non-reflectivelymeasuring the amount of light passing directly through said gap; meansfor comparing said measured value to a desired value corresponding to adesired gap width; and a controller for controlling said gap distancethrough selective linear roll displacement; wherein said adjacent rollsare positioned at a critical distance for passage of said softened andheat-plastified polymeric sheet to apply minimal stress level to thepolymeric sheet while said rolls exert a cooling effect to cool andsolidify the previously softened and heat-plastified polymeric sheet. 2.The apparatus of claim 1, wherein a plurality of gaps are present, eachwith a laser source and a laser dector, and wherein each said lasersource emits a laser beam, and said laser source and said laser detectorare located at opposite sides of said gap between said adjacent rolls.3. The apparatus of claim 1, wherein said cooling rolls are arranged ina stack which is oriented in a vertical orientation.
 4. The apparatus ofclaim 1, wherein said rolls are arranged in a cooling stack which isoriented in a horizontal orientation.
 5. The apparatus of claim 1,wherein said rolls are arranged in a cooling stack which is angledbetween a horizontal and a vertical orientation.
 6. The apparatus ofclaim 1, wherein said cooling stack comprises three cooling rolls. 7.The apparatus of claim 1, wherein said cooling stack comprises threecooling rolls, and said laser source and said corresponding laserdetector are positioned adjacent two adjacent cooling rolls, andoriented physically to measure directly said gap therebetween.
 8. Amethod of extruding a polymeric sheet, comprising the steps of: heatingand extruding a polymeric sheet having a predetermined thickness toprovide a soft and heat-plastified sheet of uniform thickness;controllably introducing said heated, soft and heat-plastified polymericsheet into a gap extending between a pair of adjacent cooling rolls;measuring the thickness of said gap directly and without reflectionbetween said adjacent cooling rolls, by aiming a laser beam into andthrough said gap to fall upon a corresponding laser detector; comparingthe measured gap thickness to a desired gap thickness; adjusting saidgap thickness by linear displacement of said cooling rolls, in responseto said measured gap thickness and said desired gap thickness; andwherein said cooling rolls are adjusted to exert minimal pressure onsaid softened and heat-plastified polymeric sheet to cool the same. 9.The method of claim 8, further comprising the step of maintaining thepressure of said rolls between about 20 and 500 pounds per linear inchof roll width throughout the sheet cooling period.